1. Field of the Invention
The present invention concerns an apparatus and a method for counting zooplankton in a liquid medium, and in particular copepods in an aqueous medium.
2. Description of Related Art
It is well known that copepods are a sort of “vitamin bomb” for fish fry. Copepods are a type of zooplankton and the nutritional value is regarded as being better than that of rotifers. The aim is to produce copepod eggs that may be harvested and purified for further sale. When used as feed to fish, the eggs are cultured and hatched, and the resulting copepods are used to feed the fish. Copepods may be used as feed for marine fish larvae in, e.g., an aquaculture facility or aquarium. Also, good results have been achieved with copepod eggs as a start-feed for the offspring of rare aquarium fish. The copepods may be of the type Acartia tonsa. 
In a large production plant for copepods for production of copepod eggs, it is desirable to provide an automated process for the egg production. Manual counting of copepod densities is time consuming, and for this reason and others, the production of live food amounts to a significant part of the production costs for marine fish species. Today's copepod plants require a human operator presence in order to distribute feed in accordance with growth and density of copepods. To enable better monitoring of copepod cultures and feed densities in larval tanks, a more efficient measurement method is needed. By exploiting the fact that copepods can be visually distinguished in size and shape from other particles present in culture water, the counting process can be automated.
An automated process for counting rotifers in culture water is schematically illustrated in FIG. 1 and described in M. O. ALVER et al.: “Automatic measurement of rotifer Brachionus plicatilis densities in first feeding tanks”; Aquacultural Engineering 36 (2007) 115-121. An imaging box 1 is provided with an object glass 2 inside. Computer controlled pumps 3 and valves 4 make it possible to automatically extract samples from one or several feeding tanks 5 into the object glass 2 inside the imaging box 1. The object glass 2 provides a defined volume at a distance from a digital camera 6. The defined volume is provided by a space between the two glass plates constituting the object glass 2. The space between the two glass plates can be chosen depending on the desired sample volume, and the focus depth of the camera. For high densities, a shorter space should be chosen to reduce the risk of rotifers overlapping in the picture. The rotifers in the volume are photographed by the digital camera and the digital image processed by image processing to obtain a rotifer density inside the fixed volume. FIG. 2 shows an arrangement of light sources 7, object glass 2 and camera lens 6 of the prior art rotifer counter shown in FIG. 1. Lighting is provided by 16 light emitting diodes 7 mounted in a square with four diodes along each side. The square is arranged so the cone visible to the camera (camera line of sight) falls in between the light emitting diodes 7. This set-up provides dark field conditions, where light is reflected by particles in the water, causing rotifers and other particles to appear in the images as bright spots against a dark background. Dark field conditions provide images with better contrast than bright field conditions.
The principles of the rotifer counter can be used for organisms that move sufficiently slowly or do not respond strongly to stimuli such as pumping or light. However, organisms such as copepodites of Acartia, and many other species, move rapidly, react to pressure gradients caused by pumping, and show strong taxis toward light. When pumping samples from a tank, this behavior invalidates the assumption that the plankton density in the measurement volume equals the density in the tank.